This adds support for separate config and data dirs.
On Linux, these will be compliant XDG Basedir Specification, i.e.
XDG_CONFIG_HOME and XDG_CONFIG_DIRS (or XDG_DATA_HOME and XDG_DATA_DIRS
respectively). On all other platforms, there currently just set to the
homedir path (so no previous behaviour has been changed).
If reicast wants to read and write a data file, it just calls
get_data_path("/samplefile.txt"). If it does not need to write to
that file, it just uses get_data_path("/samplefile.txt", false). That
way, we can also use system-wide dirs (like /usr/share/reicast on
linux), that the user usually doesn't have write access to.
The same applies for config file, where you use get_config_path(args)
respectively.
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